Heat-sensitive recording materials including an electron-donating colorless dye and an electron-accepting compound are disclosed, for example, in Japanese Patent Publication No. 14039/1970 (corresponding to British Patent 1,135,540) and Japanese Patent Publication No. 4160/1968. Heat-sensitive recording materials of this type are used in facsimile or printing out data from an electronic computer, for instance, and are advantageous, for example, in that they involve little or no maintenance requirement. However, rubbing with a nail, for instance, causes color development, which disadvantageously makes the recording face dirty; that is, fogging occurs due to friction.
Known methods for improving heat-sensitive recording materials with respect to fogging due to friction can roughly be classified into three groups.
Firstly, a method has been disclosed which comprises adding a wax to the coating mixture to thereby improve surface smoothness or slidability, and at the same time prevent the generation of the frictional heat and absorb the heat through softening of the wax. An example of the wax is polyethylene wax. See Japanese Patent Publication No. 14531/1975.
Secondly, a method is known for preventing fogging due to friction which comprises adding finely powdered starch or starch derivative to the heat-sensitive layer to thereby inhibit frictional heat generation in the neighborhood of color-developing component grains. See Japanese Patent Publication No. 5947/1976.
Thirdly, a method is known which comprises providing the heat-sensitive layer with polymer-coated particles of a substance capable of producing a decolorizing effect against the color developing materials used. Upon rubbing, the decolorizing substance oozes out and inhibits the color development. See Japanese Patent Publication (unexamined) No. 46786/1981.
These methods, however, have drawbacks. For instance, they cause decreases in density of the color developed upon use of the heat-sensitive recording material, and/or cause decreases in image retention, and hence are somewhat unsatisfactory. Moreover, they are not always satisfactory in their frictional fogging-preventing effect.